Nephropathic cystinosis is an autosomal recessive disease characterized by intra-lysosomal accumulation of the disulfide amino acid cystine. Our previous work has shown that skin fibroblasts cultured from patients with cystinosis accumulate lysosomal cystine from the degration of both exogenous and endogenous disulfide containing proteins. We propose to employ these fibroblasts to study the disulfide reduction and degradation of endogenous and exogenous proteins to determine the relative amount of lysosomal participation in the degradation of each class of proteins. These studies will require production of 35(S)-labelled rat serum albumin, and 35(S)-insulin. The results of the interaction of these uniquely labelled proteins with cystinotic fibroblasts will be analyzed both by SDS polyacrylamide gel electrophoresis, high performance liquid chromatography using coupled UV and radio-chemical detectors, and GC-mass spectrometry. This project should provide data relevant to several fundamental questions regarding cellular proteolysis from a novel perspective.